A glut of housing units and weak demand were also key reasons for the decline, the credit ratings agency said in a report.
The emirates real estate sector has been on the slide since 2014, when crude oil prices crashed, dealing a harsh blow to many Gulf investors.
Home prices dropped more than 15 per cent between then and mid-2017.The downward trajectory continued through to the end of last year, the S&P report said, with prices of residential units falling a further five to ten per cent.
S&P called the downturn a "correction" but said the sector may start to bounce back when Dubai hosts a six-month Worlds Fair in 2020.
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"We believe this correction will continue at least for this year and next, before prices stabilise in 2020 at the earliest," it said, adding that rents will likely follow the same trend.
Dubai's Expo 2020 is expected to attract up to 300,000 visitors a day when it opens in October 2020.
Dubai is slated to spend some USD 7 billion (5.7 billion euros) on infrastructure projects and USD 2.9 billion on the expansion of the metro route to the exhibition between now and events inauguration.
The property sector and related activities form around 13 per cent of Dubai's gross domestic product, which was USD 108 billion at the end of 2017.
Between December 2015 and June 2017, overseas investors put up as much as USD 41 billion to purchase property.